The present invention relates to a fixing device and image forming apparatus used to form an image on different types of paper.
In a widely used image forming apparatus, a photoreceptor is first charged, and at the same time, electric charge is removed in conformity to the image data of the document to form an electrostatic latent image, then toner is attached to the electrostatic latent image on the photoreceptor. Next, the toner attached to the photoreceptor is transferred to a transfer material such as an intermediate transfer belt or a sheet of paper to form a toner image. A fixing device is used to fix the formed toner image onto a sheet of paper.
As a fixing device, a fixing device of heat roller type which utilizes a fixing roller as a heating rotary member and a pressure roller as a pressure rotary member is used in general. In recent years, there has been an extensive use of an oilless fixing method wherein an unfixed image formed with a toner including a mold releasing agent is fixed onto paper. In commonly used fixing members, an elastic layer composed of silicone rubber or fluorine rubber is provided on the core made of aluminum and iron, and a mold releasing layer as a surface layer is formed on this elastic layer. It is a common practice to form this mold releasing layer by coating a tube made of materials having excellent release characteristics such as a fluorine resin, or by coating these materials.
The surface of the fixing roller is known to be gradually scratched due to the effect of traveling of sheets such as paper dust and toner smearing. In particular, so-called paper edge scratch is known to be produced on the surface of the fixing member by a burr present along the edge of a sheet. When a sheet is cut by a sharp-edged cutter, a burr is produced as a cutting trace of the cutter. The burr size differs according to the thickness or basis weight of paper. The burr of greater size measures approximately several to dozen microns. It should be noted that the basis weight indicates the weight as the basis of paper and paperboard expressed in terms of weight per square meter as a unit area. The unit is given in g/m2 (gsm: gram per square meter). If the sheet having a burr passes through the fixing member, a paper edge scratch is produced on the surface of the fixing member by the burr present on the edge of the paper parallel to the sheet traveling direction. And, at the time of forming an image on a sheet having a greater image formation range than the portion wherein the paper edge scratch is occurring, the paper edge scratch disturbs the toner image on the sheet and uneven brightness occurs on the image fixed on the sheet.
The Japanese Patent Application Publication No. 2008-40363 discloses the technique of minimizing the impact of the burr of paper upon the fixing member. The technique disclosed in the Japanese Patent Application Publication No. 2008-40363 is a technique of using a sliding contact member to slide on and level out the surface of the fixing member for the purpose of repairing a paper edge scratch caused by the burr of a specific sheet.
As disclosed in the Japanese Patent Application Publication No. 2008-40363,a sliding contact member for sliding the surface of a fixing member has been used in the conventional art to repair a paper edge scratch of the fixing member. This sliding contact member is capable of contacting and separating from the fixing member and is provided with recesses and protrusions to roughen, its surface. The sliding contact member is made to contact the fixing member, and the paper edge scratch on the surface of the fixing member is leveled out by the recesses and protrusions on the surface so that the fixing member is repaired, in the conventional art. When the surface roughness of the sliding contact member is increased, the paper edge scratch on the surface of the fixing member can be repaired more easily. However, a new scratch is produced on the surface of the fixing member by the roughened structure of the sliding contact member in some cases in the conventional art. In the meantime, if the surface roughness of the sliding contact member is reduced in an effort to minimize scratches caused by the sliding contact member, the paper edge scratch formed on the surface of the fixing member cannot be repaired sufficiently in some cases in the conventional art.
The technique disclosed in the Japanese Patent Application Publication No. 2008-40363 uses a sliding contact member whose surface roughness is set at a specific level with respect to the paper edge scratch formed on a specific sheet of paper. However, as described above, the size of a burr present on a sheet varies depending on the type of the paper. Thus, depending on the type of paper, a specific sliding contact member is incapable of repairing the surface of the fixing member in some cases. This has resulted in unevenness of the image fixed on the sheet, which are even visible to the user in some cases.